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Discussion in 'Shock Brother's DIY Amps' started by SngleCoil, Apr 19, 2018.
Very, very impressive work!
I vote grey as well.
I honestly don't care what color it is. I just want to hear it.
Finished enough to play out. I cut the front panel 1/8th too large all around and had to nix the piping I planned. Eventually I’ll go back and cut a smaller one because the whole thing will probably need a re-cover sooner rather than later. The vinyl I used is way too soft to survive very long.
The Lar-Mar PPIMV suits this amp very well. The “standby” switch works well enough. Mine doesn’t completely kill the signal, maybe because I’m not using a balanced PI tube. Next time I need tubes, I’ll give a balance triode in there to see if it makes a difference.
I’ll try to get a clip of it in action. I have been trying to record direct using the Two Notes Wall of Sound plugin for cab sim. It just doesn’t do justice to what I’m hearing using a real cab...but neither does my phone camera
Niiiiiiiiiiiiiiiiiiice! I like the understated look of it!
Ok amp sleuths, my amp has developed a little problem. I figured I would document here so that you fine folks who are much smarter than me can chime in with advice.
I played the amp this past weekend and it sounded great! I wanted to try a couple of different tubes in it so I ordered a balanced Sovtek 12ax7lps for the PI. As soon as I put it in and fired up the amp, there was a loud pop and a very present low level hum (very possibly 60 cycle). I replaced the original PI tube and, sadly the hum is still there.
So a couple of observations. The hum varies in intensity as the master is turned but is always there regardless of the position of the master. It is worst all the way up, all the way down, and roughly between 40-60% rotation. There is also now some scratchiness as the MV is turned. And finally, opening and closing the mute switch now pops where before it was silent.
All of this leads me to believe that I’m getting some DC past one or both the 0.047uF coupling caps coming off the PI plates. I’ll have to take some voltage readings later tonight.
Here is the schematic again for reference.
If my hunch is right...how the heck did the cap go leaky so quickly?!?
The amp was working great, then after you did a PI tube switch you heard a pop, now there's a hum for the first time, and changing the PI tube back doesn't make the hum go away.
Maybe the voltage rating of the caps is insufficient for this amp?
What are yours rated at? (the .047uF coupling caps for starters)
SoZo nextgen mustards. rated at 500V. 230ish on the plates preceding the cap. That is, of course, under normal idle load.
I have a small test amp that I’m going to pop the new tube in just to see if it works at all. Not a good comparison as the plate voltages are significantly different...but it will at least satisfy some curiosity.
Hunch confirmed...12 volts on the wrong side of one of the coupling caps. Grrr...almost makes me want to put a couple of 0.1uF in series in that position for margin.
But why a failure? Just a factory bummer? Still possible that the tube had something to do with it?
That I just don’t know, and may never. The fact that I just put a new tube in seems more than coincidental for sure. But how would a tube fail in a way that would take out the next coupling cap? Sadly I don’t yet have the experience to know.
Good news is that I have a Mallory 150 left in my parts stash and can swap it out tonight to make sure that cures the problem.
Assuming I get it fixed, and despite the fact that something went bad so soon, at least the experience gives me confidence that I can troubleshoot and fix the thing!
Could be a cold/bad/slightly-wrong solder joint on the tube socket, maybe?
The tube was the thing that was physically changed (wiggled out/wiggled in) and doing that puts some stress on the socket, and the pins on the other side. One little random/unseen hair of a wire moved and is now touching something it shouldn't...has happened to me.
Could be a cap blew on a tube swap, could happen. Has never happened to me though fwiw.
Well I can tell you what will cause a cap to fail for sure...hitting it with the side of your soldering iron when you install it
...waiting on new ones to be delivered
Nice build. Also really appreciate the effort on the schematic. You don't need software to make one, yours shows how nice a hand drawn one can look. Now hurry up and get that cap, we want to hear it.
New cap is installed and the root cause quickly revealed itself. One of the power tubes was internally shorting. That perfectly explains the pop and hum. I guess that could also explain the voltage on the wrong side the suspicious cap.
New cap in, all 4 power tubes replaced and STILL I have this wicked 60 cycle hum...but only in high power mode. Low power mode disconnects the cathodes of the outside power tubes. Makes no sense. I chopsticked around moving heater wires...no difference. One of the outside power tubes is close to the output transformer. Maybe the field from the OT is causing it to hum? I actually unscrewed the OT and had enough wire to pull it out and move it around outside the chassis. No difference. I’m totally stumped at this point.
Well, Uncle Doug to the rescue! His video on identifying hum suggested that internal capacitance in tubes can sometimes can cause 60 cycle hum. Sure enough, one of my brand new power tubes was the culprit. Thankfully by dumb luck the last time I ordered tubes, I decided to get a set of 5 matches tubes instead of 4...just in case. Popped in the replacement and the hum is gone!
In another thread recently, I posted a statement I heard from Dave Friedman. He said that modern tube manufacturing was abysmal and that they send back around 50% of the tubes they ordered because they didn’t pass testing after burn-in. I believe it! I’m getting tubes from retailers obviously, and I’m sure there is some initial QC. But 2 bad tubes out of 9 is not comforting. I guess I’m going to start paying a few bucks extra to have tubes burned in to see if that helps.
I find it more satisfying to buy 2 dozen or so output tubes at a time and match them myself, (if I have time).
So, if you have some spare time with the other 4 'matched-sets' of tubes, you could spend some time re-arranging them up to see if you can match them better.
Hook up the amp with a set of 4 multimeters at (one on) each cathode, and a couple of other meters on the plate leads on each side of the OT, and measure all 4 tubes simultaneously, noting the plate voltage, screen voltage and idle current. Do this with each set.
Then rearrange them and do the same. You'll find the readings change all over the place once you start mixing the sets up (because sets work as a 'team' and finds a new equilibrium to the other particular tubes in the set). But you will get an approximate idea which tubes burn hotter and which burn colder. Then start matching the ones at the cold end of the spectrum and the hot end etc. Then, as you get sets that match more closely, swap the tube positions around within each set until you find pairs on each side of the OT primary that are fairly evenly matched (because in a quad, the tubes on each side work in pairs and you need to match ultimately down to the pair level).
Absolutely beautiful amplifier. Congratulations on creating a work of art that will be cherished by all who play it and especially those lucky enough to see the chassis out of the cab.
You also may have taken out a screen grid resistor when the output tube shorted. Ohm-out them to check. An open Rg2 will cause hum (because that tube will not conduct, causing uneven draw on each side of the OT primary)
Sigh, So the amp is eating tubes. And after crunching some numbers, I can see why. In High power mode I'm seeing 14.97 watts plate dissipation. In low power mode it jumps to a whopping 17.95 watts . Needless to say, I red plated the tubes in low power mode. Cathode and screen resistors all measure dead on the schematic. My voltages are nearly identical to the schematic on Hoffman's site as well at the one that I took directly from my friends HC-30. I know Matchless biases these amps extremely hot but I don't know how they pull off running THIS hot.
So now...what to do about it? I guess I need to order several different values of cathode resistors to see if I can get this down to a reasonable dissipation level.
Yes, you want those EL84s to be idling between 60-80%, and for today's production tubes ideally running somewhere around 300-320 (or if you use Russian 'EL84M'/6P14P-EV/7189A you can get away with up to 400V)